1. Field of the Invention
The present invention relates to a solid state imaging apparatus, a solid state imaging device driving method and a camera and more particularly to a solid state imaging apparatus, a solid state imaging device driving method and a camera configured to realize dark signal level reduction.
2. Description of the Related Art
In a camera such as a digital still camera or a security camera, a CCD (Charge Coupled Device) type image sensor of an inter-line transfer system has been widely used.
FIG. 3 is a schematic diagram illustrating a CCD type image sensor of the inter-line transfer system. In the example shown in FIG. 3, a CCD type image sensor 101 includes a plurality of light sensing units 102 arrayed in a matrix, a read-out gate 103 disposed adjacent to each light sensing unit and configured to read out a signal charge input using the light sensing unit, a vertical transfer unit 104 disposed adjacent to the read-out gate and configured to transfer the signal charge read-out using the read-out gate in a vertical direction, a horizontal transfer unit 105 configured to transfer the signal charge transferred from the vertical transfer unit in a horizontal direction and a channel stop area (not shown in FIG. 3) disposed on the opposite side of the read-out gate of the light sensing unit and configured to reduce color mixing (for example, see Japanese Laid-Open Patent Publication No. 10-144907). The signal charge which has been transferred in the horizontal direction using the horizontal transfer unit 105 is then transferred to a floating diffusion (FD) unit 106 incorporated into an output unit. A variation in the potential of the FD unit is detected using a MOS transistor and converted into an electric signal. Then, the electric signal is amplified and output as a video image signal Vout.
Operations of the CCD type image sensor of the inter-line transfer system configured as mentioned above are classified into [1] photoelectric conversion and signal charge storage performed using the light sensing unit (a photodiode) 102, [2] transfer (field shifting) of signal charges from the light sensing unit 102 to the vertical transfer unit 104, [3] signal charge transfer (vertical transfer) performed using the vertical transfer unit 104, [4] transfer (line shifting) of signal charges from the vertical transfer unit 104 to the horizontal transfer unit 105, [5] signal charge transfer (horizontal transfer) performed using the horizontal transfer unit 105 and [6] signal charge detection and amplification performed using the FD unit 106. Next, the above mentioned operations will be described in detail.
[1] Photoelectric Conversion and Signal Charge Storage Performed Using the Light Sensing Unit (Photodiode)
An optical image which has been imaged through an imaging lens disposed on the front surface side of the CCD type image sensor 101 is converted into a charged image using the light sensing unit 102. That is, in each light sensing unit 102, a signal charge is stored in accordance with an intensity of received light and a time when the light has been received.
[2] Field Shifting
The signal charges stored in the respective light sensing units 102 are simultaneously read out to the vertical transfer unit 104 at a predetermined timing, which is referred to as “field shifting”.
Specifically, first, in an odd-numbered field, signal charges of vertically arrayed odd-numbered pixels (the light sensing units 102) are added to signal charges of vertically arrayed even-numbered pixels (the light sensing units 102) to be read out. In the next even-numbered field, by changing the combination of pixels to be added together, signal charges of vertically arrayed even-numbered pixels (the light sensing units 102) and signal charges of vertically arrayed odd-numbered pixels (the light sensing units 102) are added together and are read out. That is, two vertically adjacent pixels are added together to form one field using the vertical transfer unit 104 and then another field is formed by changing the combination of pixels to be added together, by which one frame is completed. Incidentally, such a reading method as mentioned above is referred to as a “field reading” technique which is widely adopted in the field of video cameras in order not to leave a frame afterimage behind.
[3] Vertical Transfer
The signal charges which have been field-shifted to the vertical transfer unit 104 are then vertically transferred using the vertical transfer unit 104. In the transfer using the vertical transfer unit 104, signal charges of respective lines are transferred downward (toward the horizontal transfer unit 105) in parallel in units of lines.
[4] Line-Shifting
The signal charges of one line which have been transferred to the undermost stage of the vertical transfer unit 104 are then transferred in parallel to the horizontal transfer unit 105 in the lamp, which is referred to as “line-shifting”.
[5] Horizontal Transfer
The signal charges of one line which have been line-shifted to the horizontal transfer unit 105 are then horizontally transferred using the horizontal transfer unit 105. When the horizontal transfer unit 105 changes so as to be in a vacant state with no signal charge left therein at the completion of horizontal transfer of signal charges of one line, signal charges of the next one line are line-shifted to the horizontal transfer unit 105 from the vertical transfer unit 104.
[6] Signal Charge Detection and Amplification
The signal charges which have been horizontally transferred to the left end (the terminating end) of the horizontal transfer unit 105 are detected in terms of voltages pixel by pixel and amplified using the FD unit 106 and are then output from its output terminal.
Incidentally, the above mentioned operations [1] to [6] are performed interrelated with one another. As soon as the signal charges of one line which have been vertically transferred from the undermost stage of the vertical transfer unit 104 are line-shifted to the horizontal transfer unit 105, horizontal transfer thereof is started and the signal charges are sequentially detected and read out pixel by pixel using the FD unit 106. Line shifting and vertical transfer of signal charges are performed simultaneously. That is, when all the signal charges of one line have been read out from the horizontal transfer unit 105, signal charges of the next one line which have been vertically transferred down to the undermost stage are line-shifted to the horizontal transfer unit 105 and are then horizontally transferred. By repeating the above mentioned operations, all the signal charges in one field are read out.
Photoelectric conversion is still being continuously performed while the signal charges are being transferred in the above mentioned manner and storage of signal charges is again started in respective light sensing units (photodiodes) 102 immediately after the signal charges have been field-shifted. That is, although immediately after the signal charges have been transferred to the vertical transfer unit 104 by field-shifting, each light sensing unit (photodiode) 102 changes so as to be in a vacant state, light is continuously emitted to respective light sensing units (photodiodes) and hence charges are again stored therein.
FIG. 4 is a schematic diagram for illustrating a sectional structure taken along IV-IV line of the pixel region shown in FIG. 3. On the surface of a P-type well area 119 formed on an N-type silicon substrate 110, N-type signal charge storage areas 126 constituting the light sensing unit 102, N-type charge transfer areas 124 constituting the vertical transfer unit 104 and P+-type channel stop areas 118 are formed.
A P++-type positive charge storage area 127 is formed on the surface of the signal charge storage area 126 and a P-type area interposed between the signal charge storage area 126 and the charge transfer area 124 constitutes the read-out gate 103. A gate insulating film 123 made of, for example, SiO2 is formed on the charge transfer area 124, the read-out gate 103, the positive charge storage area 127 and the channel stop area 118. A transfer electrode 125 is formed on the charge transfer area 124 via the gate insulating film 123.
Then, signal charges stored in the light sensing unit 102 are field-shifted to the adjacent vertical transfer unit 104 via the read-out gate 103. The signal charges which have been line-shifted from the vertical transfer unit 104 and then horizontally transferred from the horizontal transfer unit 105 flow into the FD unit 106 whose potential is, then, changed in accordance with the amount of stored signal charge. Incidentally, a signal (a voltage) detected at the FD unit is very small, so that a source follower circuit (constituted by a plurality of stages of MOS transistors) is connected to the FD unit 106 and the signal (voltage) detected at the FD unit 106 is amplified and output to the outside using the source follower circuit.
Incidentally, in recent digital still cameras, a tendency has been observed that the pixel size decreases with increasing resolution (increasing the number of pixels). As the pixel size decreases, (1) the sensitivity and the level of output signals such as the quantity of saturating signals are reduced and (2) the number of noise components is increased in order to obtain output signals with certainty, which may result in a decrease in SN ratio (Signal to Noise Ratio) and hence induce deterioration of image quality.
Various kinds of methods are now being developed so as to increase the sensitivity and the quantity of saturating signals with no increase in noise components and to reduce noise. In particular, from the viewpoint of noise reduction, the reduction in level of dark signals constituting most noise components in a solid state imaging device has become important.
The CCD type image sensor as mentioned above is widely used in various kinds of cameras such as security cameras and cameras for use in FA (Factory Automation) and in many cases a long-time light-exposing (sometimes, referred to as a low-speed shutter) mode is set in the cameras as mentioned above. Contrary to a “usual mode” in which field-shifting is performed per 1/60 sec in consideration of its application to a standard type TV, that is, the time period for which a signal charge is stored in the light sensing unit 102 is set to 1/60 sec, the “long-time light-exposing” mode is a mode in which signal charges are continuously stored in the light sensing unit 102 for one to two seconds or, in some case, for several seconds maximum, without performing reading-out (field-shifting) of signal charges from the light sensing unit 102. The long-time light-exposing mode is effective for performing image capture, in particular, in a dark field state. However, dark-signal-induced deterioration of image quality is noticeably observed in the long-time light-exposing mode.
As a technique for reducing the level of dark signals, a method of controlling a clock voltage applied to a vertical transfer unit and a timing at which the clock voltage is applied to the vertical transfer unit has been proposed. Next, examples of related dark signal level reducing techniques will be described with reference to the accompanying drawings.
First, FIG. 5A is a diagram showing a vertical transfer clock signal Vφ to be applied to a vertical transfer unit for a time period ranging from a light-exposing time period to a signal outputting time period in a usual situation where a dark signal number reducing technique is not used. In most commercially available CCD type image sensors, a middle bias voltage (VM) of a vertical transfer clock signal Vφ is set to 0[V] for the entire time period ranging from the light-exposing time period to the signal outputting time period. Here, a “middle bias voltage (VM)” denotes a high level potential of a clock voltage to be applied for vertical transfer of signal charges using the vertical transfer unit, that is, a high level potential set in the case that a read-out voltage is not taken into consideration. A time period D shown in FIG. 5A is a time period for which unnecessary signals generated upon light-exposure are swept.
On the other hand, as an example of the dark signal level reducing technique, a technique for negatively biasing (for example, VM=−0.5[V]) a middle bias voltage (VM) of a vertical transfer clock signal Vφ to be applied to a vertical transfer unit for the entire time period ranging from the light-exposing time period to the signal outputting time period has been proposed as shown in FIG. 5B.
In this connection, FIG. 6A is a diagram showing a relation between a middle bias voltage (VM) of a vertical transfer clock signal Vφ to be applied to a vertical transfer unit and the dark signal level. The level of dark signal is reduced by negatively biasing the middle bias voltage (VM) of the vertical transfer clock signal Vφ, so that dark signal level reduction is attained by negatively biasing the middle bias voltage (VM) of the vertical transfer clock signal Vφ to be applied to the vertical transfer unit for the entire time period ranging from the light-exposing time period to the signal outputting time period.
As another example of the dark signal level reducing technique, a technique for fixing a vertical transfer clock signal Vφ to be applied to a vertical transfer unit at a low level only for the light-exposing time period has been proposed as shown in FIG. 5C (see, for example, Japanese Laid-Open Patent Publication No. 2003-153087).